Angiography refers generally to the capture and representation of blood vessels or vasculature of the human body by means of X-ray imaging, i.e., X-ray vascular imaging. X-ray diagnostic imaging systems may be used for angiographic imaging procedures such as digital subtraction angiography (DSA), and live fluoroscopic roadmapping. Digital subtraction angiography or DSA is an imaging method used for visualizing blood vessels inside a patient's body that includes injecting a contrast medium bolus that is substantially opaque to X-rays into the blood vessels or vasculature under study as images are acquired by the X-ray diagnostic imaging system. Prior to acquisition of the contrast image, a mask image without contrast is acquired. A difference image is calculated by superimposing upon and subtracting the mask image from the contrast image. Ideally, nothing appears in the difference image other than the image of the blood vessels. Because of the time difference between acquisition of the mask image (no contrast) and acquisition of the contrast-enhanced images, global and periodic motion, fluctuations in the intensity of the X-ray source, scattering by the contrast medium, etc., unwanted artifacts may appear in the differenced or digitally subtracted angiographic image. U.S. Pat. No. 5,690,106, to Bani-Hashemi, et al. (“the '106 patent”), discloses a flexible image registration system for conducting digital subtraction angiography (DSA) with a C-arm rotational X-ray system.
Like DSA, fluoroscopic roadmapping is an angiographic imaging method for vascular imaging in which two images are overlaid to visualize blood vessels in a particular bodily area, e.g., the chest area. But unlike DSA, roadmapping includes superimposing upon and subtracting a stored 2D contrast-enhanced image (i.e., a mask image) from a current or live fluoroscopic image of the same vessel area (i.e., a non-contrast-enhanced image). Because only the stored mask image is acquired while the vessels are opacified with contrast medium the patient is generally exposed to lower levels of contrast agent as compared with a DSA study. Typically, live fluoroscopic or fluoro imaging uses lower radiation intensity as compared to DSA. U.S. Pat. No. 4,995,064 (“the '064 patent”), commonly-owned, discloses an X-ray examination apparatus that may be used for live fluoroscopic (“fluoro”) roadmapping.
Live fluoro roadmapping supports various endovascular procedures such as percutaneous transluminal coronary angioplasty, where the contrast image is superimposed on a series of live 2D fluoro images acquired while a catheter is moved through the vasculature under study. The acquired mask or contrast image frame is superimposed on the real-time non-contrast-enhanced live frames as they are acquired, and subtracted in real time. The result is a static display of the vascular structures, typically displayed in white, while the catheter appears in black. Like DSA, however, misregistration due to global and periodic motion, etc., can result in image artifacts in the subtracted live fluoro roadmapping, which degrade image quality.
To correct for misregistration, various conventional processes have developed. For example, U.S. Pat. No. 4,870,692 (“the '692 patent”) discloses a method of correcting subtraction images for patient motion in a fluoroscopy system. The method includes automatically dissecting the mask and contrast images into subregions. The pixels in those subregions are then compared using a cross-correlation correction algorithm. The cross-correlation correction algorithm ideally calculates a shift vector based on a portion of the image that has shifted by motion in one or more sub-images. The shift vector shifts the mask image to better align it with the non-contrast enhanced image or images of the same position. For each shift vector, there are several storage locations for storing its position, direction components and reliability criteria for each component.
Another known method, somewhat related to the current inventive X-ray imaging system and method for live fluoro roadmapping includes automatic sampling the acquired live images or frames (non contrast-enhanced) into the system's background process, which searches a large arbitrary region of interest (ROI) for the “best” shift vector. The sampling and searching, however, is very operation intensive and not conducive to accurate real-time correction. This is because the automatic sample, search and compare process must search the entire large region to generate the best shift vector using conventional methods. In particular, the large area for search and comparison is typically a center quarter (¼) of the image frame, or one quarter (¼) of the image area that contains the greatest amount of image or feature information.
US 2008/0027316 A1 discloses a system and a method wherein a user has to define a particular small ROI which is used for realizing a pixel shift vector.